For enhanced performance, wind turbines are sometimes provided with variable pitch blades. The pitch of such blades is adjusted by selective pivoting of the blades about their longitudinal axes, thereby enabling the wind turbine to perform at optimum efficiency in varying wind conditions, as well as aiding start-up of the wind turbine, and preventing overspeed operation of the wind turbine in high wind velocities by feathering the blades.
Various mechanisms and control systems have been proposed for varying the pitch of wind turbine blades. One such system is disclosed in U.S. Pat. No. 4,083,651 to Cheney et al. and assigned to the assignee of the present invention. This system employs pendulum members centrifugally responsive to the rotor speed of the wind turbine to twist the wind turbine blades, thereby effecting selected blade pitch throughout the operating range of the wind turbine. It will be appreciated that the blade pitch adjustment range of such a system is limited by the centrifugal response of the pendulums and is automatic, responsive only to the operating parameters of the system and therefore incapable of being overridden when desired.
To provide for the continuous control of wind turbine blade pitch, it is desirable to implement hydraulic control systems wherein hydraulic fluid is selectively supplied to and drained from hydraulic actuators operatively connected to variable pitch wind turbine blades. Hydraulic control systems operating on such a general principle, have been employed in pitch control systems for aircraft propellers, examples of such hydraulic propeller blade control systems being found in U.S. Pat. Nos. 2,611,440, 2,809,702, 3,004,608 and 3,163,233. It will be observed that the systems disclosed in these patents, generally employ a single hydraulic motor which pivots the propeller blades in unison about the longitudinal axes thereof through a single mechanical movement in normal pitch change and emergency feathering modes of operation. In such systems, hydraulic fluid from a primary pump is supplied to the motor for normal pitch change adjustment. A backup or secondary pump supplies pressurized hydraulic fluid to the motor for feathering or in the event of breakdown of the primary pump. For a number of reasons, such propeller control systems are not entirely suitable for use with the variable pitch wind turbines. By way of example, in a variable pitch wind turbine, it is generally desirable to provide the turbine with independently movable blades, a capability which is not achieved by any of the aforementioned prior art patents without duplication of the entire hydraulic system and mechanical movement disclosed therein. Such duplication would of course be economically prohibitive and detract significantly from the efficiency of the turbine by adding significant weight to the turbine hub. Furthermore, it is desirable to provide redundancy in that portion of the pitch control system which controls emergency blade feathering, thereby minimizing the risk of damage to the turbine due to a malfunction of one of the feather control portions of the system. Such redundancy in the feather control system portion is clearly unattainable with the systems disclosed in the aforementioned patents without further duplication of those systems. Moreover such redundant feather control portions of the system should be capable of being hydraulically isolated from one another so that a malfunction of one feather control system due to, for example, depressurization from leakage, will not adversely affect the other or backup feather control system. The propeller blade pitch control systems of the aforementioned patents are inappropriate for and unadaptable to such independent and redundant feather control.
Other aircraft propeller control systems are disclosed in U.S. Pat. Nos. 2,505,206, 2,556,700 and 2,507,671. These systems, like those discussed hereinabove, generally provide pitch change adjustment and feathering of a plurality of aircraft propeller blades in unison. Unlike the systems of the aforementioned patents, the systems of these prior art patents employ accumulators or passive hydraulic fluid sources as a backup for a primary pump for blade feathering. However, it will be appreciated that utilization of an accumulator does not in any way cure the unadaptability of these systems to wind turbine blade pitch and feather control wherein independent pitch movement of each blade and hydraulically isolated fluid sources for feathering are required.